Plug-in detection is a feature commonly included in computer systems and data communications systems, in which a host computer or similar device is capable of detecting whether the far end of a data communications cable is plugged in to a peripheral or similar device. In some types of data communications systems, the host device only begins transmitting data over a cable in response to it detecting that the far end of the cable has been plugged in to an appropriate peripheral device.
Plug-in detection systems commonly detect whether a cable having a copper conductor has been plugged in by sensing the impedance presented to the host device via the cable. When the electrical connector at the far end of the cable is not plugged in to a peripheral device, a higher impedance is presented to the host device via the cable than when the electrical connector is plugged in to the peripheral device. The plug-in detection system in the host device commonly includes a charging resistor and a coupling capacitor that, in conjunction with the above-referenced impedance, forms a resistor-capacitor (R-C) circuit that the plug-in detection system can cause to charge and discharge. The R-C time constant is slower when the cable is plugged in to a peripheral device than when the cable is not plugged in to the peripheral device. Additional circuitry in the plug-in detection system can sense a change in this time constant. When the plug-in detection system senses that this R-C time constant has decreased below a threshold, the plug-in detection system outputs a signal indicating that the cable has been plugged in. Other circuitry in the host device may respond to this plug-in detection signal by initiating tasks such as transmitting data via the cable.
The above-described type of plug-in detection system is operable with cables having copper signal conductors. It is not readily adaptable to cables that carry signals over optical fibers.
An active optical cable (AOC) is a data communications cable having electrical connectors and electrical-to-optical and optical-to-electrical converters at its ends (typically within housings that serve as the cable plugs), with optical fibers extending between those converters. Thus, an electrical signal received at a first electrical connector at one end of the AOC is converted into an optical signal, conveyed through the optical fiber, converted back to an electrical signal, and provided to a second electrical connector at the other end of the AOC. Some AOCs are bidirectional and include both an electrical-to-optical and an optical-to-electrical converter at each end of the AOC, with a first optical fiber carrying signals in one direction and a second optical fiber carrying signals in the opposite direction.